Chemical silver bath

ABSTRACT

Baths for the electroless deposition of silver on a metal, glass, ceramic or plastic surface comprise an alkaline silver cyanide solution, a reducing agent, a brightening agent, and a compound of sulfur and/or selenium acting as an accelerator.

United States Patent [1 1 Ludwig et al.

[ Oct. 28, 1975 1 CHEMICAL SILVER BATH [75] Inventors: Rolf Ludwig; Karl-Hans Fuchs, both of Berlin, Germany [73] Assignee: Schering AG, Berlin, Germany [22] Filed: Aug. 8, 1973 [21] Appl. No.: 386,637

Related [1.8. Application Data [63] Continuation-in-part of Ser. No. 186,549, Oct. 4,

1972, abandoned.

[52] US. Cl 106/1; 117/35 S; 117/130 E [51] Int. Cl. C23C 3/02 [58] Field of Search 106/1; 204/46 R;

[56] References Cited UNITED STATES PATENTS 2,883,288 4/1959 Dobbs et a1 t. 106]] 2,976,180 3/1961 Brookshire 106/] 3,492,135 l/l970 Claus 106/1 FOREIGN PATENTS OR APPLICATIONS 1,058,915 2/1967 United Kingdom 106/] OTHER PUBLICATIONS A. P. C. App. of Weiner, Ser. No. 351,241, 5/18/43.

Primary Examiner-Lorenzo B. Hayes Attorney, Agent, or FirmJoseph F. Padlon 57 ABSTRACT 2 Claims, No Drawings CHEMICAL SILVER BATH CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 186,549, filed Oct. 4, 1972, now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to baths for the electroless deposition of silver on metal or metallized substrates, which make possible greatly accelerated rates of deposition, while avoiding the drawbacks of previously known baths.

Baths for the deposition of silver on a surface, without the use of an electric current from an external source are known. It is also known that non-metallic as well as metallic materials can be silver-plated without the use of electric current.

For this purpose, alkaline baths are used. These baths generally contain a silver salt, a complex former, a reducing agent, and various additives for regulating the rate of deposition, for hardening the metal film, and for increasing the adherence of the deposit. Such baths are disclosed, for example, in British Pat. Nos. 661,270 and 776,508, French Pat. No. 1,218,178, and U.S. Pat. No. 2,801,935.

A particular disadvantage of the known silver baths is their content of ammonia, which, as is known, may lead to the formation of the explosive fulminate of silver. Another disadvantage of the known baths is that the properties of the silver coatings deposited therefrom are unsatisfactory.

Accordingly, the art has sought an ammonia-free bath which permits the deposition of strongly adherent silver coatings in a dense and compact form. The object of the present invention was to provide such baths, and this object is achieved by the present invention.

GENERAL DESCRIPTION OF THE INVENTION In accordance with the present invention there are provided novel electroless silver plating baths containing silver in the form of silver cyanide, which baths are alkaline, contain free cyanide ions, and which contain, as an accelerator of deposition, at least one compound of sulfur and/or selenium, or of sulfur combined with selenium. The baths of the invention may also include conventional additives, such as reducing agents, buffering compounds, brightening agents, complexing agents, wetting agents and the like.

Particularly suitable are those sulfur and selenium compounds in which the sulfur or the selenium has a degree of oxidation minus one or minus two." By degree of oxidation is meant herein the so-called oxidation number or charge value, i.e., the charge which an atom in a molecule would have if the latter were built up only of ions. Sulfur or selenium compounds in which sulfur or selenium is at the oxidation stage minus one" or minus two" are capable of greatly accelerating the rate of deposition of silver from the baths of the invention.

It is known to employ various selenium compounds as stabilizers in electroless copper plating baths, and such baths and plating methods are disclosed, for example, in US. Pat. No. 3,492,135. However, in copper plating baths the selenium compounds produce little or no accelerating effect. It could not therefore have been predicted that similar sulfur and selenium compounds would have a quite different activity in alkaline silver cyanide plating baths.

DETAILED DESCRIPTION OF THE INVENTION The sulfur and/or selenium compounds employed in the practice of the present invention may be either inorganic or organic. They are known compounds and can be produced by known methods. Such methods are described in l-Iouben-Weyl, Methoden der organisehen Chemie, Volume 9 (1955).

The sulfur or selenium additives of the invention are employed in silver baths in concentrations from about 0.0001 to 0.5 moles per liter of bath liquid, the compounds being added either alone or in admixture with one another.

Suitable additive sulfur or selenium compounds having the above-mentioned degrees of oxidation fall into the following groups, having the general formulas indicated:

1. Compounds of the general formula wherein R, and R are identical or different and signify hydrogen, a univalent metal equivalent or an organic radical; R signifies in addition the groups CN or SO Me, Me being a metal atom, and X, and X are identical or different and signify a sulfur or a selenium atom.

11. Compounds of the general formula wherein R, and R are identical or different and signify hydrogen, a univalent metal equivalent, or an organic radical; R signifies in addition the groups CN or SO Me, Me being a metal atom, and X is a sulfur or a selenium atom.

III. Compounds of the general formula wherein R," and R are identical or different and signify a univalent metal equivalent or an organic radical; R," signifying in addition hydrogen; R is an organic radical; X is a sulfur or a selenium atom; and Z is an acid radical.

IV. Compounds of the general formula wherein R," and R are identical or different and signify an organic radical; X is a sulfur or a selenium atom; and Y is a nonmetal atom.

It should be noted that the metallic disulfides and diselenides, wherein R, and R represent a metal atom, are predominantly ionogenic, so that the formulae should more accurately be written for example as R,R X or as R, R (XX). This is known to those skilled in the art.

As univalent metals there enter into consideration sodium, potassium, or calcium/2, and the like. Examples of suitable organic radicals include those which are aliphatic, cycloaliphatic, araliphatic, and aromatic, which may be optionally substituted and/or interrupted by one or more hetero atoms, such as oxygen, nitrogen or sulfur and/or one or more hetero atom groups, of the following types:

For the onium compounds the usual acid radicals of onium compounds may be employed, such as the radicals of the inorganic acids, preferably of the hydrogen halide acids.

Compounds which may be employed according to the invention include the following:

pounds, such as polyacrylic acid, polyacryl acetate and polyacrylamide, polyvinyl alcohol, carboxymethyl cellulose, carboxyethyl cellulose, polyethylene glycols, stearic polyglycol ether, or the copolymers of the foregoing substances with other polymerizable compounds, such as acrolein, methyl vinyl ketone, vinyl sulfonic acid, N-vinyl pyrrolidone, N-vinyl carbazole, or vinyl pyridine. The molecular weight of these compounds may range from about 200 to 100,000, preferably from about 500 to 10,000. Also, they can be used in concentrations from about 0.00001 to 0.1 mole per liter.

The foregoing compounds are known and can be produced by known methods, such as described in Houben-Weyl, Methoden der organischen Chemie, Volume 14/1 (1961) and 14/2 (1963).

Inorganic compounds suitable for improving the rate of deposition are chiefly silicates, phosphates and borates, which are especially effective in concentrations of about 0.001 to 0.5 mole per liter.

The bath may also contain commonly used constituents, such as reducing agents, bases, and optionally wetting and brightening agents.

The source of the silver to be deposited is a silver salt. Examples of suitable silver salts to be incorporated into the baths of the invention include silver nitrates or These compounds are known in the art and can be produced by known methods or slight modifications of known methods, as described in Houben-Weyl, loc. cit.

The bath should contain, besides, free cyanide ions in concentrations of about 0.01 to about 1.0 mole per liter, preferably 0.1 to 0.4 mole per liter. These are added advantageously in the form of the alkali cyanides, such as sodium or potassium cyanide.

It is also advantageous for increasing the rate of deposition to add to the plating bath a polymeric organic or condensed inorganic compound containing at least one hydroxyl group. One or more such compounds may be used. Instead of the compounds containing hydroxyl groups, it is of course possible also to use those compounds from which hydroxyl groups are formed in the alkaline bath by hydrolysis of suitablegroups, such as the nitrile, ester or amide groups.

Examples of suitable organic polymeric compounds containing a hydroxyl group include polyacrylic comother water soluble silver compounds, such as silver nitrite, silver fluoride, silver chlorate, silver perchlorate, silver acetate or silver sulfate, which are complexed in the bath solution by the cyanide ions. Moreover, silver cyanide or already complexed silver salts, such as potassium dicyano-argentate, may be used.

As reducing agents, there may be employed, for example, hydrazine and its derivatives, hypophosphites, such as sodium hypophosphite, boranates, such as sodium tetrahydridoborate, and formaldehyde. These are used in concentrations of from about 0.1 to about 1.0 mole per liter.

To adjust the alkaline range, there are added bases, preferably alkali hydroxides.

The basic composition of the baths according to the invention it therefore:

Silver as metal 0.01 0.25 gram-atom/liter Free cyanide ions 0.05 0.3 mole/liter Alkali hydroxide 0.1 2.5 mole/liter Reducing agent 0.1 |.0 mole/liter If desired, still other additives may be incorporated into the bath, such as metal compounds of metals of the third and fourth main groups and of the second and eighth sub-groups in the Periodic Table of the elements, if it is desired to deposit alloys of silver with these metals.

The following are examples of metal compounds suitable for this purpose: thallium nitrate, sodium hexahydroxostannate, mercury nitrate, palladium chloride, all of which are added in concentrations of about 0.001 to 1.0 mole per liter.

The method of utilizing the baths according to the invention comprises immersing the part to be plated, which has been properly pretreated according to the base material, in the solution. It is advantageous either to agitate or to move the stock and, for the purpose of attaining high rates of deposition and smooth surfaces, to employ continuous circulation, and possibly also filtration, of the bath. The operating temperatures will range from about 75 to about 95C, preferably about 60C.

The baths of the invention are particularly adapted for the chemical silver plating of metallic surfaces such as copper, silver, gold, nickel and brass. However, after proper pretreatment nonmetallic surfaces, such as glass, ceramics or plastics, can also be metallized.

It is especially noteworthy that the baths of the present invention operate at a very high rate of deposition of up to 20 microns per hour. This represents a considerable technical advance, since silver plating of catalytically active surfaces in accordance with the invention becomes possible in a short time. Another advantage of the baths according to the invention resides in that the silver content of the bath can be exhausted almost completely without having the rate of deposition fall below an acceptable value.

In this connection, there is also provided the possibility of intensification or correction, in that there may be added to the bath, for example, the missing quantity of potassium dicyanoargentate with the proportion of a metal compound, e.g. a zinc salt, which regulates the cyanide content.

The various sulfur and selenium compounds of the invention listed at pages 5-6 above are all water soluble. Thus, for example, compound No. '10, sulfolan selenocyanate is miscible with water in all proportions, thus being completely soluble. This compound is used in Example 4 below in the amount of 0.002 mole perliter, which is approximately equal to 448 mg., i.e., about 0.5 g/l.

In the absence of the sulfur and/or selenium compounds according to the invention, the alkaline cyanide plating baths are short-lived, and theirusefulness is limited. Thus, after to minutes, decomposition of the baths takes place, i.e., silver is spontaneously reduced, not only on to the substrate which is to be plated, but within the body of the bath solution itself, and is indeed virtually uncontrollable. As a consequence thereof, there are formed on the substrate spongy nonadherent deposits of silver.

The surprising and unexpected character of the invention whereby the sulfur and/or selenium compounds act as deposition accelerators in the case of silver plating baths, although the selenium compounds act only as stabilizers in conjunction with electroless copper plating baths of the type shown in US Pat. No. 3,492,135, is shown by the following comparative tests in which the speed of deposition of copper from the bath of copper sulfate containing the disodium salt of ethylenediamine tetra-acetic'acid, sodium hydroxide, and formaldehyde, as described in the several examples 5 of said U.S. patent, is compared with the speed of deposition from a chemical silver plating bath of the general type disclosed in the examples below, and containing potassium dicyano argentate, potassium cyanide, potassium hydroxide, and hydrazine hydrate. The same selenium compound, namely bis(potassium carboxyethyl) diselenide, was added to each of the copper and silver plating baths. This additive is disclosed in said U.S. patent at columns 34, line 13, and in this specification as compound 12, at page 6.

Test No. l

A chemical copper plating bath was prepared having the following composition: 1

A metal coupon was immersed in the bath in three runs, and the velocity of deposition was measured. The average of three measurements was 2.9 micron per hour.

Test No. 2

A silver chemical plating bath was prepared having the following composition:

Potassium dicyano argentate Potassium cyanide Potassium hydroxide Hydrazine hydrate Bis( potassium carboxyethyl) diselenide 0.|5 mole-(29.9 g)/liter 0.l mole (6.5 g)lliter 1.2 mole (67.3 g)/liter 0.25 mole (12.5 g)/liter 0.005 mole/liter A metal coupon was immersed in the bath in three runs, and the velocity of deposition was measured. The average of three measurements was 9.3 micron per hour.

It will be apparent that the selenium compound which acts simply as a stabilizer in the copper bath causes a rate of deposition in the silver bath which is more than threefold that in the copper bath.

Further comparative tests were made using similar copper and silver plating baths, but employing the compound potassium benzyl thiosulfate as the additive. This compound is the sulfur analog of the compound potassium benzyl selenosulfate shown as compound 6 in the list on page 5 above.

Test No. 3

A chemical copper plating bath was prepared having the following composition:

Potassium benzyl thiosulfate 0.004 g/liter 7 A metal coupon was immersed in the bath in three runs, and the velocity of deposition was measured. The average of three measurements was 2.8 micron per hour.

Test No. 4

A chemical silver plating bath was prepared having the following composition:

0.1 mole( 19.9 g)/liter 0.04 mole (2.6 g)/liter 0.8 mole(44.0 g)/liter Potassium dicyanoargentate Potassium cyanide Potassium hydroxide Formaldehyde 0.45 mole( 13.5 g)/liter Potassium benzyl thiosulfate 0.01 mole (2.4 g)/liter A metal coupon was immersed in the bath in three runs, and the velocity of deposition was measured. The average of three measurements was 15.6 micron per hour.

Thus, the sulfur compound causes a rate of deposition from the silver bath which is more than five times that from the copper bath.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Silver nitrate 0.08 mole/liter Potassium cyanide 0.23 Potassium thiocyanate 0.03 Sodium hydroxide 1.25 Polyacrylamide 0.01 Hydrazine hydrate 0.40 Bath temperature 95C Example 2 Potassium dicyanoargentate 0.10 mole/liter Potassium cyanide 0.08 Potassium selenocyanate 0.01 Potassium hydroxide 1.08 I Polyacrylic acid 0.05 Sodium hypophosphite 0.50 Bath temperature 100C Example 3 Potassium dicyanoargentate 0.15 mole/liter. Potassium cyanide 0.10 Dipropyl diselenide 0.005 Potassium hydroxide 1.2 Potassium silicate 0.1 I Hydrazine hydrate 0.25 Bath temperature 85C Example 4 Silver cyanide 0.10 mole/liter Potassium cyanide 0.15 Sulfolan selenocyanate 0.002 U Sodium hydroxide 1.0 Polyacryl acetate 0.006 Sodium tetrahydridoborate 0.5 Bath temperature 75C -Continued Example 5 Potassium dicyanoargentate 0.108 mole/liter Potassium cyanide 0.05 Thiourea 0.001 Potassium hydroxide 1.6 Polyvinyl alcohol 0.004 Hydrazine hydrate 0.45 Bath temperature 65C Example 6 Silver nitrate 0.10 mole/liter Potassium cyanide 0.43 Mercury nitrate 0.05 Potassium thiocyanate 0.02 mole/liter Potassium hydroxide 1.0 Polyethylene glycol 0002 Hydrazine hydrate 0.45 Bath temperature 95C Example 7 Potassium dicyanoargentate 0.09 mole/liter Potassium cyanide 0.03 Potassium hexahydroxo UV) stannate 0.05

Potassium thiocyanate 0.01 Potassium hydroxide 0.80 Sodium metaphosphate 0.08 Sodium hypophosphite 0.45 Bath temperature 100C Example 8 Potassium dicyanoargentate 0.| mole/liter Potassium cyanide 0.04 Thallium nitrate 0.04 Potassium benzyl selenosulfate 0.01

Potassium hydroxide 0.8 Vinyl sulfonic acid 0.02 Formaldehyde 0.45 Bath temperature C Example 9 Potassium dicyanoargentate 0.1 mole/liter Potassium cyanide 0.06 Palladium chloride (11) 0.03 Sodium thiosulfate 0.01 mole/liter Potassium diselenide 0.005 Potassium hydroxide 1.20 Sodium metaborate 0.1 Sodium tetrahydridoborate 0.3 Bath temperature 95 C What is claimed is:

1. In an alkaline aqueous electroless reducing bath for the deposition of silver, said bath containing silver ions, cyanide ions, and a reducing agent capable of reducing said ions to metallic silver on the surface of an article immersed therein, the improvement which consists in the presence in said bath as a deposition accelerator of at least one compound of sulfur, selenium, or sulfur and selenium, in which the sulfur or selenium is in the oxidation state minus 1 or minus 2, selected from the group consisting of diethyl sulfane, methylpropyl selenosulfane, potassium diselenide, bis(p-potassium sulfophenyl diselenane), potassium benzyl selenosulfate, potassium benzyl thiosulfate, propyl selenocyanate, sulfolan selenocyanate, tribenzyl sulfonium bromide, and tribenzyl selenonium bromide, in an amount effective to accelerate the deposition and to form a dense compact adherent deposit of metallic silver on said article, being between about 0.0001 and about 0.5 moles per liter of bath.

2. The bath of claim 1 in which the concentration of cyanide ion is between about 0.1 and about 1.0 mole per liter of bath. 

1. IN AN ALKALINE AQUEOUS ELECTROLESS REDUCING BATH FOR THE DEPOSITION OF SILVER, SAID BATH CONTAINING SILVER IONS, CYANIDE IONS, AND A REDUCING AGENT CAPABLE OF REDUCING SAID IONS TO METALLIC SILVER ON THE SURFACE OF AN ARTICLE IMMERSEID THEREIN, THE IMPROVEMENT WHICH CONSISTS IN THE PRESENCE IN SAID BATH AS A DEPOSITION ACCELERATOR OF AT LEAST ONE COMPOUND OF SULFUR, SELENIUM, OR SULFUR AND SILENIUM, IN WHICH THE SULFUR OR SELENIUM IS IN THE OXIDATION STATE MINUS 1 OR MINUS 2, SELECTED FROM THE GROUP CONSISTING OF DIETHL SULFANE, METHYLPROPL SELENOSULFANE, POTASSIUM DISELENIDE, BIS(P-POTASSIUM SULFOPHENYL DISELENANE), POTASSIUM BENZYL SELENOSULFATE, POTASSIUM BENZYL THIOSULFATE, PROPYL SELENOCYANATE, SULFOLAN SELENOCYANATE, TRIBENZYL SULFONIUM BROMIDE, AND TRIBENZYL SELENONIUM BROMIDE, IN AN MOUNT EDFFECTIVE TO ACCELERATE THE DEPOSITION AND TO FORM A DENSE COMPACT ADHERENT DEPOSIT OF METALLIC SILVER ON SAID ARTICLE, BEING BETWEEN ABOUT 0.0001 AND ABOUT 0.5 MOLES PER LITER OF BATH.
 2. The bath of claim 1 in which the concentration of cyanide ion is between about 0.1 and about 1.0 mole per liter of bath. 